Important Links for Homework Problems
- An oscillating dipole (How to visualize the electrical field of a diple in space, and how it changes as the dipole oscillates)
- A good story to recapitulate the first part of electricity and magnetism
- How does a battery
work? By Ionic and Electronic Conduction
(This well-made site from Duracell explains the structure, chemistry, working and history of batteries.
The animation showing electronic currents and the mechanism of ionic conduction requires to download "Shockwave", but is positively tops.) - Electronic Conduction in
Electrical Circuits: Visualize Electronic Motion in Solids
(Requires the download "Shockwave") - What exactly is the structure
of matter? An animation primer on the structure of the atom
(Requires the download "Shockwave") - The
Periodic Table of the Elements
They are several resources on the net, of which I recommand several:- The Chemicool Periodic Table of Elements
from the MIT-Tech
(This table is in my opinion one of the best. It has a lot of technical data in easy to read table form, from density to melting point, which you access by clicking on the desired element.) - If the MIT site is busy...
(Here is another excellent web-based periodic table, due to its clarity.) - Periodic Table with Comic
Relief
(The following site illustrates the "character" of each element with a comic or cartoon. For those of you who have less innate passion for chemistry.)
- The Chemicool Periodic Table of Elements
from the MIT-Tech
- A
recap on Rotational Dynamics
(A nice slide show for the University of Illinois at Urbana-Champagne which review important concepts, those pesky vectors, the meaning of formulations and some of the concept-tests done in class.) - An in-depth recapitulation on dielectrics, E-field and potential in capacitors for the UK
Recent Assignments from Prof. Herbots
- Homeworks
- Recitations
- Homework/Recitation
- Homework Solutions
- Discussion on Recitation#4
- Homework#19
- Last Recitation/Extra Credit Homework#25
Grade Distribution for Exam #1 (9/8/200) BEFORE extra-credit
| GRADE DISTRIBUTION | ||||||
2 |
11 |
14 |
15 |
29 |
21 |
9 |
| E | D | C | B | A | A+ | A++ |
| >38 | >48 | >58 | >68 | >78 | >88 | >98 |
| Page 1 | Page 2 | Page 3 | Page 4 | Page 5 | Total w/o XC | |
| Average in% | 0.98 |
0.90 |
0.65 |
0.56 |
0.54 |
0.69 |
| Standard Deviation in % | 0.10 |
0.16 |
0.21 |
0.27 |
0.35 |
0.15 |
ACEPT Physics PHY 131 |
Prof. N. Herbots/TA J. Shaw |
Mid-term # 2 - Version 4University Physics II: Electricity & Magnetism E-field, Gauss's law, Potential, Energy in a circuit Friday, 10/8/2000 |
Your television and computer monitor are essentially:
L = 40 cm
- (40 pts) Propose a simple, spherically
symmetric model for a doubly charged phosphorus ion P+2 ( = a P atom having lost 2 electrons) and compare it to the
P atom by drawing the charge distributions for (a) the atom and (b) the ion, (c&d) the
resulting Efield inside and outside BOTH the atom and the ion.
HINT: Consider very simply that all electronic shells are spherical and have the same
radius R =0.1 nm (a nanometer =1 nm = 10-9 m). The nucleus has a radius
Rn = 10 fm( a femtometer = 10-15 meter)
MODEL IN WORDS: DRAWING (DRAW the E-field Inside and outside the atom and the ion)- ATOM
- ION
- (30 pts) Based on your model, what would be the magnitude of
the electrical field you have drawn at a distance of 1 nm from the center of the ion and
the atom. (a) Be sure to show and explain whether and why it would be zero or not. (b) If
it is not zero, find an expression for the Efield,
and explain. (c) Calculate the value for the P2+ ion an
- E-field outside the atom
- E-field outside the ion
- Value for the E-field outside the ion at a distance R = 1 nm from the center
- (50 pts) The phosphorus ions are really part of bigger molecules. The figure on the page shows what the charge configuration looks like in a phosphorus oxide compound present on your screen: the P2+ ions form an equilateral triangle. The spacing between the Phosphorus and oxygen ions is d = 1 nm. Draw on the molecule below (a) the magnitude of the electrical field vectors Efield at the location marked by the lettters A, B, C, D, E, F, G, H (b) draw the field lines everywhere (c) the force vector on an electron at the location marked "electron" Explain how you proceed to find the (d) net Efield and (e) the fieldlines
(30 pts) Model the electron gun as a negative point charge, and the screen as a flat, positively charged sheet . (a) Is there an electrical field inside that tube? (b) If so, draw the electrical field vectors inside the TV tube between the electron gun and the screen here below. (c) Explain your reasoning next to the drawing.
(a) Graph the electrical potential energy Epe , the kinetic energy KE and the total energy for the electron as a function of the distance x between the gun and the screen. Is energy conserved?
(b) Mark the 4 locations where Epe and KE are maximum and minimum.
(c) As the electrons travel from the gun to the screen, explain what happens to the Epe and KE?
(d) Graph the electrical potential Ve(x) as a function of the distance x between the gun and the screen by making a dashed line for the electrical potential Ve(x) on the same graph.
(e) Given that the value of the electrical potential difference DVe on page 1between the electron gun and the screen, calculate the difference in D Epe of an electron between the gun and the screen when the TV is on.
(f)Based on your answers in (a -e), you can very easily deduce the magnitude of the E-field between the electron gun and the center of the screen for a TV monitor of total length L. How?
(e) Given the value of L on page 1, calculate the numerical value of the E-field,
- Show in a drawing what happens to the metallic cylinder supporting the tip of the antenna when it is immersed in the E-field of the broadcasting signal at a fixed time t (this is an electrostatic "snapshot")
- Describe and calculate the Efield inside and outside the antenna. Be complete and show how to derive EACH Efield affecting the metallic cylinder, and how they combine inside and outside.
- Explain what would happen if the cylinder is grounded. (Be sure to make a drawing to support your explanation).
- Based on you answer to question #2, graph and give an expression for the magnitude of (a) the electrical field, (b) the electrostatic potential energy (c) the electrostatic potential as a function of the radial distance from the center nucleus for BOTH the ion and the atom. Be sure to graph these quantities inside and outside the inner radius. Be sure to show how the three quantities are related.
- Where is the electrostatic potential energy highest?
Prof. N. Herbots/TA J. Shaw |
Mid-term # 1 - Version 4University Physics II: Electricity & Magnetism Electrostatics Friday, 9/8/2000 |
Aluminum
foil
You have built several electroscopes in recitations. The standard electroscope shown above and built in recitation #2, consists of a thin strip of aluminum folded into two and hanging from a plastic straw.
You charge a glass or a plastic stick by friction and approach it from the electroscope.
In this exam, you will build a electrostatic model and make calculations about the electroscope you are already familiar with it.
For starter, just describe in a few words what you observe when you approach a charged object to the aluminum strip. The next questions will probe into more details your understanding of this effect.
QUESTION #2: The atoms forming the materials- Describe briefly in words and sketch an electrical model for
an atom (In other words, what kind of charges are present in an aluminum atom, how many
and how are they distributed?)
Give a MODEL IN WORDS: and DRAWING - Now, you have charged the stick by friction. The charge
collected on the stick is negative. (a) This time, explain with a
complete model what happens to charges when the stick comes close but does not touch
the aluminum foil. Be sure to explain the behavior of all charges present! (b)
Include the occurrence of polarization if you think it is taking place, (c)
indicate where polarization would be relevant to the problem , (d) be sure to show
and explain what polarization is. (e) Label and explain clearly which materials are
insulators and which are conductors, and (f) what the differences are between the
two. BE COMPLETE. Point out where you chose to make simplifying assumptions.
Use WORDS and DrawingS
| QStick = Q | QStick = Q | QStick = 4Q |
| mAl = 1 gram | mAl = 2 gram | mAl = 1 gram |
QUESTION #4: Deduce the magnitude of the electrostatic force
Using your analysis and results for the model in question #3 and given that mAl = 1 gram- guess or estimate the angle of deflection in the picture of page 1 (or use your own recollection).
- calculate the magnitude of the electrostatic force.
- Felectrostatic (formulation and
calculations, units) =
DRAW the electrostatic vector forces on the picture in page1, (b) making sure that you draw vectors with the correct relative magnitude.
explain briefly your reasoning: Question#6: Deduce the field
- Define the concept electrical field and how it is calculated.
- Draw the electrical field at the tip of the stick near the foil.
- Calculate the magnitude of the electrical field.
- EXTRA CREDIT: Sketch the electrical field vectors for all directions around the stick.
Aluminum foil
Question#7: Improve the model
You were asked to consider the charges as point charges. (a) Now refine your model taking into account a more realistic charge distribution. Explain your reasoning. (b) Explain how this would change your calculation of the electrostatic forces and the charge. (c) Showing how and where your formulations and force diagrams would be modified, discuss whether the particle model overestimates or underestimates the force and the charges. Use words, drawings and formulations in your explanation. Question#8: Let's try one possible charge distribution. Assume that the charge of the stick is now distributed uniformly along the stick, from the tip to the location where your hand is holding it.- Draw a pictorial representation of the charge distribution and the force diagram on or next the figure below.
- Using this charge distribution, set up the integral to
calculate the net electrostatic force on the stick. To make it easy, ASSUME that the stick
is held perfectly horizontal, pointing straight towards the tip of the aluminum foil.
Model all other charges as point charges.
Be sure to show clearly the geometry, all your work and all steps.
Felectrostatic =
- Solve the force integral
- Compute the value of force via this integral for the case when you are holding the stick 1 cm from the tip, assuming the charge you computed in Question # 5 is a good estimate.. Compare that value to what you calculated in Question # 4 for Felectrostatic
- Represent and calculate the Felectrostatic between the silicon and the two oxygens. Explain all steps
- Represent and calculate the net dipole moment p of the molecule. Be sure to explain what a dipole moment is and why there would be zero or non-zero dipole moment on this molecule.
